diff --git a/news/subprocess-completions-dir-spaces.rst b/news/subprocess-completions-dir-spaces.rst
new file mode 100644
index 000000000..eebb28791
--- /dev/null
+++ b/news/subprocess-completions-dir-spaces.rst
@@ -0,0 +1,27 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* Subprocess-based completions like
+  `xontrib-fish-completer <https://github.com/xonsh/xontrib-fish-completer>`_
+  no longer append a space if the single available completion ends with
+  a directory separator. This is consistent with the behavior of the
+  default completer.
+
+**Security:**
+
+* <news item>
diff --git a/xonsh/completers/tools.py b/xonsh/completers/tools.py
index 48ddeb908..cdb369bd2 100644
--- a/xonsh/completers/tools.py
+++ b/xonsh/completers/tools.py
@@ -275,7 +275,7 @@ def complete_from_sub_proc(*args: str, sep=None, filter_prefix=None, **env_vars:
             lines = output.split(sep)
 
         # if there is a single completion candidate then maybe it is over
-        append_space = len(lines) == 1
+        append_space = len(lines) == 1 and not lines[0].rstrip().endswith(os.sep)
         for line in lines:
             if filter_prefix and (not filter_func(line, filter_prefix)):
                 continue